The invention relates to a process for the manufacture of cellulosic fibers from a solution of cellulose in a tertiary amine oxide and possibly water, whereby the solution formed into fibers through a spinneret is coagulated in at least two stages and the fibers are subsequently washed and dried; and to cellulosic fibers.
A process for the manufacture of cellulosic fibers from a solution of cellulose in a tertiary amine oxide and water, which are also known as Lyocell or solvent-spun fibers, is described for example in U.S. Pat. No. 4,246,221. In this so-called Lyocell process, cellulose is dissolved in an organic solvent such as N-methylmorpholine-N-oxide (NMMO). The solution, which may also contain water and possibly a stabilizer such as gallic acid propyl ester, is extruded through a spinneret into an air gap to form fibers or filaments and then coagulated in a coagulation bath. Following the coagulation bath is a withdrawal component such as a galette, over which the fibers are guided under tension. With the help of further galettes the fibers are transported on to the next treatment steps. These are usually fiber washing, finishing, drying and winding up.
Lyocell fibers exhibit a strong tendency to fibrillate. WO95/30043, WO96/07777, WO96/07779 and EP-A-0 691 426 propose measures for reducing the tendency of Lyocell fibers to fibrillate. These measures comprise the addition of additives to the coagulation agent, the use of special gases in the air gap or the post-treatment of the fibers with chemicals such as crosslinking agents. However, these methods have the drawback that in view of ensuring that the process is performed in an environmentally-friendly manner, the chemicals additionally introduced into the process have to be recovered by special methods, as a result of which the processes become more difficult and expensive.
WO96/20301 also discloses a process for the manufacture of formed cellulose objects such as fibers or filaments from a solution of cellulose in a tertiary amine oxide. The fibers made according to this publication, which are also claimed to have a reduced tendency to fibrillation, have a core-sheath structure. In the core of the fibers there is a highly ordered hypermolecular configuration with small, finely dispersed pores and in the sheath there is a relatively unordered hypermolecular configuration with large heterogeneous cavities. The core-sheath structure of the fibers is achieved by guiding the fibers formed from the solution through at least two coagulation baths, one after the other, whereby in the first coagulation bath the cellulose is coagulated more slowly than in the final coagulation bath. For this purpose, the first coagulation bath might be an alcoholic bath such as hexanol or a mixture of hexanol and isopropanol. In the second coagulation bath an aqueous NMMO might be used, whereby the first coagulation bath is arranged directly above the second coagulation bath. This process for manufacturing core-sheath fibers, too, exhibits the drawback that additional chemicals have to be introduced to the process. These additional substances get into the washing water of the baths following coagulation, along with the tertiary amine oxide used to prepare the solution.
The Lyocell process is known to be particularly environmentally friendly since the tertiary amine oxide used to prepare the solution can be almost completely recovered and returned to the solution preparation process. The use of other chemical substances makes this recovery more difficult and is thus detrimental to the economic efficiency of the process.